1. Field of the Invention
This invention relates to a control system for ignition timing and boost pressure in an internal combustion engine and more particularly to a control system for ignition timing and boost pressure in a multicylinder internal combustion engine equipped with a turbocharger.
2. Description of the Prior Art
In the conventional multicylinder internal combustion engine equipped with a turbocharger, the ignition timing is set the same for all cylinders of the engine. As a result, when knocking occurs, it is eliminated by retarding the ignition timing of all the cylinders, and when the retardation comes to exceed a prescribed amount, the boost pressure is then reduced. This is the technique used in, for example, Japanese Laid-Open Patent Application No. 58(1983)-167881.
As a multicylinder internal combustion engine equipped with a turbocharger is subject to a large thermal load, it is highly susceptible to damage caused by knocking. It is therefore necessary to monitor the state of combustion in the engine combustion chambers so as to detect the occurrence of knocking and, when knocking is detected, to eliminate it as quickly as possible. At the same time, however, unless the engine is operated at an ignition timing which is as close as possible to the knocking limit, it is not possible for the turbocharger to produce the increase in power output for which it was installed, greatly reducing the significance of its installation. Moreover, the conditions under which knocking occurs are not generally the same for all cylinders of the engine and it is frequently found that knocking occurs in one and the same cylinder only. In the prior art technique, since the ignition timing is the same for all cylinders, retardation of the ignition timing to eliminate knocking results in retardation of the ignition timing in all cylinders, including those in which no knocking has occurred. Then when the retardation comes to exceed the prescribed amount, the boost pressure is reduced. As a result, the engine output is lowered more than necessary. Moreover, when the ignition timing is retarded, the period during which combustion is possible in the combustion chambers is shortened and this results in unburned air/fuel mixture being passed into the exhaust system, where it burns and increases the temperature of the exhaust gases. In the conventional technique, since the ignition timing is retarded for all cylinders even though knocking has occurred in only one, the amount of unburned gas generated becomes proportionately larger, resulting in an increase in exhaust temperature and a corresponding increase in the risk of damage to the engine. Also, as the conventional system determines the boost pressure reduction timing solely on the basis of the amount of ignition retardation implemented for elimination of knocking, it has been criticized for its inability to realize engine control which directly reflects changes in engine operating conditions or changes in the operating environment.